Windows Telephony Services – 2025 Patch Diffing And Analysis Part 2

In the pre­vi­ous part 1 We dis­cussed the win­dows tele­pho­ny ser­vices and how it works, So we can go on and per­form patch diff­ing. Now, In this part (Part 2), we delve into the patch diff­ing and analy­sis of the 2025 se­cu­ri­ty up­dates for Mi­crosoft Tele­pho­ny Ser­vices. Af­ter out­lin­ing the ar­chi­tec­tur­al and op­er­a­tional as­pects of TAPI and its re­lat­ed com­po­nents in Part 1, we now turn our fo­cus to the de­tailed ex­am­i­na­tion of the code changes that ad­dress the heap-based buffer over­flow vul­ner­a­bil­i­ties. This sec­tion ex­plores how spe­cif­ic mod­i­fi­ca­tions to mem­o­ry man­age­ment rou­tines, func­tion calls, and er­ror-han­dling mech­a­nisms serve to mit­i­gate po­ten­tial RCE. Let’s Go a head and Per­form Our patch diff­ing.

Lab Set­up

We will be us­ing Ghidra, Bin­Ex­port, and Bin­Diff. You can down­load them from the fol­low­ing links:

• Ghidra
• Bin­Ex­port Ghidra Plu­g­in
• Bin­Diff

The Patch

There are many ex­cel­lent re­sources ex­plain­ing Mi­crosoft’s patch­ing process, MSU files, and Win­dows patch diff­ing in gen­er­al. Here are some use­ful ref­er­ences:

• In­tro­duc­tion to Bi­na­ry Diff­ing – Part 1
• In­tro­duc­tion to Bi­na­ry Diff­ing – Part 2
• In­tro­duc­tion to Bi­na­ry Diff­ing – Part 3
• Win­dows Patch Diff­ing – Richard Os­good
• Ex­tract­ing and Diff­ing MS Patch­es in 2020
• YouTube: Bi­na­ry Diff­ing Tu­to­r­i­al
• YouTube: Patch Diff­ing Deep Dive

For this analy­sis, I ob­tained two ver­sions of the Tele­pho­ny ser­vice files: tapi32.dll and tapi3.dll. You can down­load the vul­ner­a­ble and patched ver­sions from this repos­i­to­ry.

In­stalling Bin­Ex­port

Next, we need to in­stall the Bin­Ex­port plu­g­in for Ghidra.

In the extension.properties file in­side the Bin­Ex­port fold­er, en­sure the version match­es your in­stalled Ghidra ver­sion. Then, com­press the fold­er into a .zip file.

Open Ghidra, nav­i­gate to File > In­stall Ex­ten­sions, and click the + but­ton to se­lect the BinExport.zip file.

Fi­nal­ly, en­able the check­box to ac­ti­vate the plu­g­in. Now we’re ready for the next step.

Cre­at­ing a Pro­ject

Cre­ate a new pro­ject in Ghidra (you can name it af­ter the rel­e­vant CVE) and im­port the DLL files.

Open the files in the Code Brows­er. When prompt­ed to an­a­lyze the file, se­lect No for now.

Next, nav­i­gate to Edit > Sym­bol Serv­er Con­fig and add Mi­crosoft’s sym­bol serv­er as a trust­ed and en­abled source.

Fi­nal­ly, run Analy­sis > Auto An­a­lyze and click An­a­lyze. Re­peat these steps for all files.

Patch Diff­ing with Bin­Diff

Now, we’ll use Bin­Diff to com­pare the files and iden­ti­fy changes.

In the Ghidra pro­ject win­dow, right-click each file and se­lect Ex­port > Bi­na­ry­Ex­port for Bin­Diff.

Af­ter ex­port­ing, you should see the .BinExport files in your di­rec­to­ries:

Directory: C:\Users\azima\Desktop\Telephony Services\patched

Mode                 LastWriteTime         Length Name

—-                 ————-         —— —-

-a—-          4/1/2025   8:29 AM        1228447 tapi32_patched.dll.BinExport

-a—-          4/1/2025   8:29 AM        3515858 tapi3_patched.dll.BinExport

Directory: C:\Users\azima\Desktop\Telephony Services\unpatched

Mode                 LastWriteTime         Length Name

—-                 ————-         —— —-

-a—-          4/1/2025   8:30 AM        1160198 tapi32_unpatched.dll.BinExport

-a—-          4/1/2025   8:31 AM        3232800 tapi3_unpatched.dll.BinExport

An­a­lyz­ing tapi3.dll

Open Bin­Diff and cre­ate a new work­space (File > New Work­space).

Nav­i­gate to Diffs > New Diff and se­lect the un­patched ver­sion as the pri­ma­ry file and the patched ver­sion as the sec­ondary file. Click Diff and wait for the analy­sis to com­plete.

Once loaded, dou­ble-click the diff (1), go to Matched Func­tions (2), and sort by sim­i­lar­i­ty (3).

You’ll see a list of func­tions that have changed be­tween the two ver­sions.

Dou­ble-click­ing a func­tion (e.g., Growbuf) re­veals the dif­fer­ences in as­sem­bly code.

In the pri­ma­ry (un­patched) view, the code per­forms cer­tain op­er­a­tions, while the sec­ondary (patched) view shows new blocks—like­ly se­cu­ri­ty checks. The next step is to an­a­lyze these mod­i­fied func­tions to iden­ti­fy the vul­ner­a­bil­i­ties.

Tapi3.dll Patch Diff­ing (Ghidra)

The fol­low­ing is the list of the func­tion names we need to per­form patch diff­ing on:

1. Grow­Buf
2. lineGath­erDig­itsW­Post­Process
3. phoneDe­vSpeci­fic­Post­Process
4. lineDe­vSpeci­fic­Post­Process
5. lineDe­vSpe­cif­ic
6. phoneDe­vSpe­cif­ic
7. lineCre­ateAgentSes­sion
8. Re­size­Call­Params
9. li­neGet­De­v­Con­fig
10. lineGath­erDig­its
11. li­neGe­tA­gent­Caps
12. Pho­neGet­Dis­play
13. SavedAdress­Name
14. li­neGetQueueListA
15. ParkIndi­rect
16. li­neGet­Grou­pList
17. GetID
18. Do­Func
19. Ini­tial­ize
20. GetID
21. Up­date­In­fo
22. Con­fig­Di­alogEd­it
23. Get­De­v­Con­fig
24. LinePark
25. put_But­ton­Text
26. Pho­neGetID
27. Li­neGetID
28. Up­date­In­fo
29. AsyncEventsThread
30. Up­date­In­fo
31. In­ter­nal­Cre­ateAgent
32. Ini­tial­ize
33. Reg­is­ter­Call­No­ti­fi­ca­tions
34. NewOb­ject
35. Cre­ate­Han­dleTableEn­try

1. Grow­Buf

• Un­patched

/* int __cdecl GrowBuf(unsigned char * __ptr64 * __ptr64,unsigned long * __ptr64,unsigned
   long,unsigned long) */


int __cdecl GrowBuf(uchar **param_1,ulong *param_2,ulong param_3,ulong param_4)


{
  uint uVar1;
  uint uVar2;
  uchar *_Dst;
  
  uVar2 = *param_2;
  uVar1 = uVar2 + param_4;
  do {
    uVar2 = uVar2 * 2;
  } while (uVar2 < uVar1);
  _Dst = (uchar *)ClientAllocReal(uVar2);
  if (_Dst != (uchar *)0x0) {
    memcpy(_Dst,*param_1,(ulonglong)param_3);
    ClientFree((longlong)*param_1);
    *param_1 = _Dst;
    _Dst = (uchar *)0x1;
    *param_2 = uVar2;
  }
  return (int)_Dst;
}

The un­patched GrowBuf func­tion dy­nam­i­cal­ly ex­pands a mem­o­ry buffer while pre­serv­ing its con­tents—ide­al for re­siz­able ar­rays or file buffers. It cal­cu­lates the re­quired size by dou­bling the cur­rent ca­pac­i­ty un­til it fits the new data, al­lo­cates a larg­er block us­ing ClientAllocReal, and copies ex­ist­ing data be­fore free­ing the old buffer. If suc­cess­ful, it up­dates the buffer point­er and size, re­turn­ing 1; if al­lo­ca­tion fails, it re­turns 0. This ap­proach en­sures ef­fi­cient mem­o­ry growth (avoid­ing fre­quent re­al­lo­ca­tions) while main­tain­ing data in­tegri­ty, mak­ing it per­fect for sce­nar­ios like stream­ing data or dy­nam­i­cal­ly grow­ing struc­tures where the fi­nal size is un­known up­front.

• Patched

/* int __cdecl GrowBuf(unsigned char * __ptr64 * __ptr64,unsigned long * __ptr64,unsigned
   long,unsigned long) */


int __cdecl GrowBuf(uchar **param_1,ulong *param_2,ulong param_3,ulong param_4)


{
  uint uVar1;
  bool bVar2;
  uchar *_Dst;
  uint uVar3;
  ulonglong uVar4;
  
  bVar2 = wil::details::FeatureImpl<>::__private_IsEnabled
                    (&`private:_static_class_wil::details::FeatureImpl<>&___ptr64___cdecl_wil::Featu re<>::GetImpl(void)'
                      ::__l2::impl);
  uVar3 = *param_2;
  if (bVar2) {
    if (((uVar3 != 0) && (uVar4 = (ulonglong)uVar3 * 2, uVar4 < 0x100000000)) &&
       (uVar1 = uVar3 + param_4, uVar3 <= uVar1)) {
      do {
        uVar3 = (uint)uVar4;
        if ((uVar1 <= uVar3) && (param_3 <= uVar3)) goto LAB_18004ee90;
        uVar4 = (uVar4 & 0xffffffff) * 2;
      } while (uVar4 < 0x100000000);
    }
  }
  else {
    uVar1 = uVar3 + param_4;
    do {
      uVar3 = uVar3 * 2;
    } while (uVar3 < uVar1);
LAB_18004ee90:
    _Dst = (uchar *)ClientAllocReal(uVar3);
    if (_Dst != (uchar *)0x0) {
      memcpy(_Dst,*param_1,(ulonglong)param_3);
      ClientFree((longlong)*param_1);
      *param_1 = _Dst;
      *param_2 = uVar3;
      return 1;
    }
  }
  return 0;
}

The patched GrowBuf func­tion be­gins by query­ing a WIL fea­ture flag through wil::details::FeatureImpl<>::__private_IsEnabled, which de­ter­mines whether to use a more se­cure but com­pu­ta­tion­al­ly in­ten­sive growth al­go­rithm or fall back to the stan­dard ex­po­nen­tial growth ap­proach. When the fea­ture is en­abled, the func­tion im­ple­ments care­ful ca­pac­i­ty dou­bling with 32-bit over­flow pro­tec­tion (ex­plic­it­ly check­ing against the 0x100000000 bound­ary) and ad­di­tion­al val­i­da­tion that the new size can ac­com­mo­date both ex­ist­ing data (param_3) and re­quest­ed ex­pan­sion (param_4). The over­flow-pro­tect­ed path uses 64-bit arith­metic (ulon­glong) for in­ter­me­di­ate cal­cu­la­tions while main­tain­ing 32-bit bound­aries for the fi­nal al­lo­ca­tion size. In the tra­di­tion­al growth path, the func­tion re­verts to sim­ple ex­po­nen­tial dou­bling un­til the ca­pac­i­ty meets re­quire­ments. Both paths con­verge at the al­lo­ca­tion point us­ing ClientAllocReal, fol­lowed by the crit­i­cal se­quence of mem­cpy for data preser­va­tion, ClientFree for old buffer deal­lo­ca­tion, and point­er/size up­dates. The func­tion’s re­turn val­ue pro­vides sim­ple but cru­cial feed­back about op­er­a­tion suc­cess (1) or fail­ure (0). This im­ple­men­ta­tion demon­strates sev­er­al ad­vanced mem­o­ry man­age­ment con­cepts: con­di­tion­al al­go­rithm se­lec­tion based on run­time fea­ture flags, arith­metic over­flow pro­tec­tion, 64-bit-safe cal­cu­la­tions for 32-bit al­lo­ca­tions, and main­te­nance of the strong ex­cep­tion safe­ty guar­an­tee (ei­ther the op­er­a­tion com­pletes ful­ly or leaves orig­i­nal state un­changed).

Grow­Buf Sum­ma­ry

So, What we can see from here is that GrowBuf func­tion con­tained a crit­i­cal in­te­ger over­flow vul­ner­a­bil­i­ty where dou­bling the buffer size (uVar2 * 2) could wrap around 32-bit lim­its, po­ten­tial­ly caus­ing heap cor­rup­tion through un­der­sized al­lo­ca­tions and dan­ger­ous mem­cpy op­er­a­tions. The patched ver­sion in­tro­duces mul­ti­ple se­cu­ri­ty im­prove­ments: (1) a WIL fea­ture flag to tog­gle safe/lega­cy modes, (2) 64-bit arith­metic with ex­plic­it 4GB bound­ary checks to pre­vent over­flow, (3) val­i­da­tion en­sur­ing the new size ac­com­mo­dates both ex­ist­ing and new data, and (4) main­tained back­ward com­pat­i­bil­i­ty through fall­back log­ic. This patch ef­fec­tive­ly mit­i­gates mem­o­ry cor­rup­tion risks by pre­vent­ing in­te­ger over­flow dur­ing size cal­cu­la­tions while adding run­time-con­fig­urable safe­ty checks, trans­form­ing a vul­ner­a­ble buffer ex­pan­sion mech­a­nism into a ro­bust, se­cu­ri­ty-hard­ened op­er­a­tion suit­able for pro­duc­tion sys­tems. The key dif­fer­ence lies in the patched ver­sion’s sys­tem­at­ic pre­ven­tion of arith­metic over­flow edge cas­es that could be ex­ploit­ed for ar­bi­trary mem­o­ry writes.

In the next section, we will be covering functions 2, 3 and 4 together as the approach is the same.

2, 3 & 4 lineGath­erDig­itsW­Post­Process, phoneDe­vSpeci­fic­Post­Process & lineDe­vSpeci­fic­Post­Process

The phoneDevSpecificPostProcess, lineDevSpecificPostProcess & lineGatherDigitsWPostProcess have the same way of patch­ing.

• Un­patched

/* void __cdecl lineGatherDigitsWPostProcess(struct _ASYNCEVENTMSG * __ptr64) */


void __cdecl lineGatherDigitsWPostProcess(_ASYNCEVENTMSG *param_1)


{
  void *_Dst;
  
  if (g_bLoggingEnabled != 0) {
    TRACELogPrint(0x80002,"lineGatherDigitsWPostProcess: enter");
    if (g_bLoggingEnabled != 0) {
      TRACELogPrint(0x40002,"\t\tdwP1=x%lx, dwP2=x%lx, dwP3=x%lx, dwP4=x%lx",
                    (ulonglong)*(uint *)(param_1 + 0x18),(ulonglong)*(uint *)(param_1 + 0x1c),
                    *(undefined4 *)(param_1 + 0x20),*(undefined4 *)(param_1 + 0x24));
    }
  }
  if (((byte)param_1[0x18] & 0x1b) != 0) {
    _Dst = (void *)GetHandleTableEntry(*(ulong *)(param_1 + 0x1c));
    DeleteHandleTableEntry(*(ulong *)(param_1 + 0x1c));
    memcpy(_Dst,param_1 + 0x38,(ulonglong)*(uint *)(param_1 + 0x24) * 2);
  }
  *(undefined4 *)(param_1 + 0x20) = 0;
  *(undefined4 *)(param_1 + 0x1c) = 0;
  return;
}

lineGatherDigitsWPostProcess func­tion han­dles asyn­chro­nous dig­it col­lec­tion events in what ap­pears to be a Win­dows tele­pho­ny ser­vice. The func­tion ac­cepts a _ASYNCEVENTMSG struc­ture point­er con­tain­ing dig­it col­lec­tion pa­ra­me­ters and per­forms three key op­er­a­tions: di­ag­nos­tic log­ging, con­di­tion­al data pro­cess­ing, and state cleanup. The log­ging sys­tem first checks the glob­al g_bLoggingEnabled flag and, if en­abled, out­puts de­bug traces in­clud­ing the func­tion en­try point and four DWORD pa­ra­me­ters from spe­cif­ic off­sets (0x18, 0x1c, 0x20, 0x24) in the mes­sage struc­ture. The core pro­cess­ing log­ic ex­am­ines bit flags in the first pa­ra­me­ter (off­set 0x18) us­ing mask 0x1B (bi­na­ry 00011011), which like­ly cor­re­sponds to tele­pho­ny event flags like LINE_CALL­STATE_SPE­CIF­IC (0x01), LINE_CALL­STATE_BLOCKED (0x10), and LINE_CALL­STATE_AC­CEPT­ED (0x08). When these flags are ac­tive, the func­tion per­forms crit­i­cal mem­o­ry op­er­a­tions: it re­trieves a mem­o­ry han­dle from off­set 0x1c via GetHandleTableEntry (part of Win­dows’ han­dle man­age­ment sys­tem), im­me­di­ate­ly deletes the han­dle from the table (trans­fer­ring own­er­ship), and copies Uni­code dig­it data from off­set 0x38 in the mes­sage struc­ture to the re­trieved buffer lo­ca­tion. The copy op­er­a­tion uses a dy­nam­ic size cal­cu­la­tion (val­ue at off­set 0x24 mul­ti­plied by 2 for Uni­code char­ac­ters), but no­tably lacks bounds check­ing on ei­ther the source or des­ti­na­tion buffers. The func­tion con­cludes by ze­ro­ing out fields at off­sets 0x20 (pos­si­bly a sta­tus field) and 0x1c (the han­dle field), ef­fec­tive­ly re­set­ting the mes­sage state. From a se­cu­ri­ty per­spec­tive, sev­er­al con­cerns emerge: the unchecked mem­cpy op­er­a­tion could lead to buffer over­flows if ma­li­cious or cor­rupt­ed mes­sages are processed, the han­dle man­age­ment as­sumes prop­er syn­chro­niza­tion that might not ex­ist in mul­ti-thread­ed sce­nar­ios, and the bit­mask val­i­da­tion pro­vides in­com­plete pro­tec­tion against mal­formed mes­sages.

• Patched

/* void __cdecl lineGatherDigitsWPostProcess(struct _ASYNCEVENTMSG * __ptr64) */


void __cdecl lineGatherDigitsWPostProcess(_ASYNCEVENTMSG *param_1)


{
  bool bVar1;
  uint uVar2;
  void *_Dst;
  ulonglong uVar3;
  ulong uVar4;
  ulong local_res10 [2];
  void *local_res18;
  ulonglong local_res20;
  
  if ((g_bLoggingEnabled != 0) &&
     (TRACELogPrint(0x80002,"lineGatherDigitsWPostProcess: enter"), g_bLoggingEnabled != 0)) {
    TRACELogPrint(0x40002,"\t\tdwP1=x%lx, dwP2=x%lx, dwP3=x%lx, dwP4=x%lx",
                  (ulonglong)*(uint *)(param_1 + 0x18),(ulonglong)*(uint *)(param_1 + 0x1c),
                  *(undefined4 *)(param_1 + 0x20),*(undefined4 *)(param_1 + 0x24));
  }
  if (((byte)param_1[0x18] & 0x1b) != 0) {
    uVar4 = 0;
    local_res10[0] = 0;
    bVar1 = wil::details::FeatureImpl<>::__private_IsEnabled
                      (&`private:_static_class_wil::details::FeatureImpl<>&___ptr64___cdecl_wil::Fea ture<>::GetImpl(void)'
                        ::__l2::impl);
    if (bVar1) {
      _Dst = (void *)GetHandleTableEntry(*(ulong *)(param_1 + 0x1c),local_res10);
      uVar4 = local_res10[0];
    }
    else {
      _Dst = (void *)GetHandleTableEntry(*(ulong *)(param_1 + 0x1c));
    }
    local_res18 = _Dst;
    DeleteHandleTableEntry(*(ulong *)(param_1 + 0x1c));
    uVar2 = *(uint *)(param_1 + 0x24);
    uVar3 = 0;
    local_res10[0] = uVar2;
    bVar1 = wil::details::FeatureImpl<>::__private_IsEnabled
                      (&`private:_static_class_wil::details::FeatureImpl<>&___ptr64___cdecl_wil::Fea ture<>::GetImpl(void)'
                        ::__l2::impl);
    if (bVar1) {
      uVar3 = (ulonglong)uVar2 * 2;
      local_res20 = uVar3;
      if (0xffffffff < uVar3) {
LAB_180051599:
        *(undefined4 *)(param_1 + 0x20) = 0;
        *(undefined4 *)(param_1 + 0x1c) = 0;
        if (g_bLoggingEnabled == 0) {
          return;
        }
        TRACELogPrint(0x40002,"lineGatherDigitsWPostProcess: It has failed due to Corrupted pMsg");
        return;
      }
      uVar2 = (uint)uVar3 + 0x38;
      if (((uVar2 < 0x38) || (*(uint *)param_1 < uVar2)) || (uVar4 < (uint)uVar3))
      goto LAB_180051599;
    }
    memcpy(_Dst,param_1 + 0x38,uVar3 & 0xffffffff);
  }
  *(undefined4 *)(param_1 + 0x20) = 0;
  *(undefined4 *)(param_1 + 0x1c) = 0;
  return;
}

the patched lineGatherDigitsWPostProcess func­tion be­gins with di­ag­nos­tic log­ging (con­trolled by g_bLoggingEnabled) that records the mes­sage’s four DWORD pa­ra­me­ters at off­sets 0x18 (event flags), 0x1c (han­dle ref­er­ence), 0x20 (sta­tus), and 0x24 (Uni­code char­ac­ter count). The crit­i­cal se­cu­ri­ty en­hance­ments man­i­fest in the con­di­tion­al pro­cess­ing path where the 0x1b bit­mask (LINE_CALL­STATE_SPE­CIF­IC|LINE_CALL­STATE_AC­CEPT­ED|LINE_CALL­STATE_BLOCKED) trig­gers dig­it buffer han­dling. The mem­o­ry op­er­a­tions now use a bi­fur­cat­ed ap­proach based on WIL’s FeatureImpl sta­tus. When en­abled, it uti­lizes an aug­ment­ed GetHandleTableEntry that out­puts al­lo­ca­tion meta­da­ta to local_res10, while the lega­cy path main­tains back­ward com­pat­i­bil­i­ty. The rev­o­lu­tion­ary im­prove­ment comes in the buffer size val­i­da­tion: the Uni­code char­ac­ter count (from off­set 0x24) un­der­goes 64-bit ex­pan­sion (ulon­glong) be­fore dou­bling, with ex­plic­it over­flow check­ing against MAXD­WORD (0xF­FFF­FFFF). Three ar­chi­tec­tur­al safe­guards then val­i­date: (1) the com­put­ed buffer end ad­dress (0x38 + size*2) doesn’t wrap be­low 0x38, (2) the to­tal doesn’t ex­ceed the mes­sage struc­ture’s de­clared size (first DWORD), and (3) the han­dle table en­try’s al­lo­ca­tion (uVar4) ac­tu­al­ly con­tains the re­quired ca­pac­i­ty. The mem­cpy op­er­a­tion now uses san­i­tized pa­ra­me­ters (bit­masked to 32-bit via &0xf­fff­ffff) only af­ter pass­ing all checks. Any fail­ure trig­gers a se­cured exit path (LAB_180051599) that nul­li­fies sen­si­tive fields (0x1c, 0x20) and logs cor­rup­tion warn­ings. This rep­re­sents a text­book ex­am­ple of mod­ern Win­dows se­cu­ri­ty hard­en­ing – main­tain­ing COM-style bi­na­ry com­pat­i­bil­i­ty while in­ject­ing ro­bust mem­o­ry pro­tec­tions through fea­ture-flagged val­i­da­tion lay­ers. The so­lu­tion el­e­gant­ly ad­dress­es three key vul­ner­a­bil­i­ty class­es: in­te­ger over­flows (through 64-bit arith­metic), buffer over­flows (via bounds check­ing), and use-af­ter-free risks (with han­dle state val­i­da­tion).

phoneDe­vSpeci­fic­Post­Process, lineDe­vSpeci­fic­Post­Process & lineGath­erDig­itsW­Post­Process Sum­ma­ry

So, what we can ob­serve here is that the orig­i­nal lineGatherDigitsWPostProcess func­tion con­tained dan­ger­ous mem­o­ry safe­ty vul­ner­a­bil­i­ties where an unchecked memcpy op­er­a­tion us­ing *(uint *)(param_1 + 0x24) * 2 could lead to buffer over­flows if the size val­ue was cor­rupt­ed, while also risk­ing in­te­ger over­flows that might cause un­der­sized al­lo­ca­tions. (From the patched code we can say it’s with-in pMsg) The patched ver­sion in­tro­duces cru­cial se­cu­ri­ty en­hance­ments: (1) WIL fea­ture flags to en­able ro­bust val­i­da­tion lay­ers, (2) 64-bit arith­metic with ex­plic­it over­flow checks when cal­cu­lat­ing Uni­code buffer sizes, (3) three-tier bounds val­i­da­tion en­sur­ing the copy op­er­a­tion stays with­in both source and des­ti­na­tion buffer lim­its, and (4) se­cure han­dle man­age­ment with al­lo­ca­tion size ver­i­fi­ca­tion. This up­date ef­fec­tive­ly elim­i­nates mem­o­ry cor­rup­tion risks by sys­tem­at­i­cal­ly pre­vent­ing arith­metic over­flows, buffer bound­ary vi­o­la­tions, and in­valid han­dle deref­er­ences, while main­tain­ing back­ward com­pat­i­bil­i­ty through con­di­tion­al fea­ture ac­ti­va­tion.

5. lineDe­vSpe­cif­ic

• Un­patched

LONG __stdcall
lineDevSpecific(HLINE hLine,DWORD dwAddressID,HCALL hCall,LPVOID lpParams,DWORD dwSize)


{
  ulong uVar1;
  ulong uVar2;
  LONG extraout_EAX;
  undefined auStack_e8 [32];
  undefined4 local_c8 [2];
  ulonglong local_c0;
  ulonglong local_b8;
  ulonglong local_b0;
  ulonglong local_a8;
  ulonglong local_a0;
  LPVOID local_98;
  ulonglong local_90;
  undefined8 local_88;
  undefined8 uStack_80;
  undefined8 local_78;
  undefined8 uStack_70;
  undefined8 local_68;
  undefined8 uStack_60;
  undefined4 local_58;
  undefined2 local_54;
  undefined local_52;
  undefined8 local_51;
  undefined local_49;
  ulonglong local_48;
  
  local_48 = __security_cookie ^ (ulonglong)auStack_e8;
  uVar1 = CreateHandleTableEntry((__uint64)lpParams);
  local_c8[0] = 0xd0057;
  uVar2 = GetFunctionIndex(lineDevSpecificPostProcess);
  local_c0 = (ulonglong)uVar2;
  local_90 = (ulonglong)dwSize;
  local_51 = 0;
  local_49 = 0;
  local_88 = 0;
  uStack_80 = 0;
  local_58 = 0;
  local_78 = 0;
  uStack_70 = 0;
  local_54 = 0x600;
  local_68 = 0;
  uStack_60 = 0;
  local_52 = 9;
  local_b8 = (ulonglong)hLine;
  local_b0 = (ulonglong)dwAddressID;
  local_a8 = (ulonglong)hCall;
  local_a0 = (ulonglong)uVar1;
  local_98 = lpParams;
  uVar2 = DoFunc((_FUNC_ARGS *)local_c8);
  if ((int)uVar2 < 1) {
    DeleteHandleTableEntry(uVar1);
    mapTAPIErrorCode(uVar2);
  }
  else {
    WaitForReply(uVar2);
  }
  __security_check_cookie(local_48 ^ (ulonglong)auStack_e8);
  return extraout_EAX;
}

The lineDevSpecific func­tion es­tab­lish­es a se­cure com­mu­ni­ca­tion chan­nel be­tween user-mode callers and tele­pho­ny hard­ware dri­vers by first cre­at­ing a pro­tect­ed han­dle table en­try for the in­put pa­ra­me­ters via CreateHandleTableEntry. This han­dle man­age­ment sys­tem con­verts raw point­ers into ref­er­ence-count­ed ker­nel ob­jects, pre­vent­ing di­rect mem­o­ry ac­cess and pro­vid­ing iso­la­tion be­tween user-mode and ker­nel-mode com­po­nents. The func­tion then con­structs a metic­u­lous­ly or­ga­nized 232+ byte stack frame that serves as a se­cure mes­sage pack­et, con­tain­ing both con­trol meta­da­ta (in­clud­ing the op­er­a­tion code 0xD0057 and a post-pro­cess­ing call­back in­dex ob­tained through GetFunctionIndex) and tele­pho­ny con­text (with all han­dles safe­ly con­vert­ed to 64-bit val­ues to pre­vent trun­ca­tion at­tacks).

• Patched

LONG __stdcall
lineDevSpecific(HLINE hLine,DWORD dwAddressID,HCALL hCall,LPVOID lpParams,DWORD dwSize)


{
  bool bVar1;
  uint uVar2;
  ulong uVar3;
  LONG extraout_EAX;
  undefined auStack_e8 [32];
  undefined4 local_c8 [2];
  ulonglong local_c0;
  ulonglong local_b8;
  ulonglong local_b0;
  ulonglong local_a8;
  ulonglong local_a0;
  LPVOID local_98;
  ulonglong local_90;
  undefined8 local_88;
  undefined8 uStack_80;
  undefined8 local_78;
  undefined8 uStack_70;
  undefined8 local_68;
  undefined8 uStack_60;
  undefined4 local_58;
  undefined2 local_54;
  undefined local_52;
  undefined8 local_51;
  undefined local_49;
  ulonglong local_48;
  
  local_48 = __security_cookie ^ (ulonglong)auStack_e8;
  bVar1 = wil::details::FeatureImpl<>::__private_IsEnabled
                    (&`private:_static_class_wil::details::FeatureImpl<>&___ptr64___cdecl_wil::Featu re<>::GetImpl(void)'
                      ::__l2::impl);
  if (bVar1) {
    uVar2 = CreateHandleTableEntry((__uint64)lpParams,dwSize);
  }
  else {
    uVar2 = CreateHandleTableEntry((__uint64)lpParams);
  }
  local_c8[0] = 0xd0057;
  uVar3 = GetFunctionIndex(lineDevSpecificPostProcess);
  local_58 = 0;
  local_c0 = (ulonglong)uVar3;
  local_90 = (ulonglong)dwSize;
  local_51 = 0;
  local_49 = 0;
  local_88 = 0;
  uStack_80 = 0;
  local_54 = 0x600;
  local_78 = 0;
  uStack_70 = 0;
  local_52 = 9;
  local_68 = 0;
  uStack_60 = 0;
  local_b8 = (ulonglong)hLine;
  local_b0 = (ulonglong)dwAddressID;
  local_a8 = (ulonglong)hCall;
  local_a0 = (ulonglong)uVar2;
  local_98 = lpParams;
  uVar3 = DoFunc((_FUNC_ARGS *)local_c8);
  if ((int)uVar3 < 1) {
    DeleteHandleTableEntry(uVar2);
    mapTAPIErrorCode(uVar3);
  }
  else {
    WaitForReply(uVar3);
  }
  __security_check_cookie(local_48 ^ (ulonglong)auStack_e8);
  return extraout_EAX;
}

The Patched lineDevSpecific func­tion rep­re­sents a se­cu­ri­ty-hard­ened tele­pho­ny in­ter­face that im­ple­ments a so­phis­ti­cat­ed dual-mode ar­chi­tec­ture for han­dling de­vice-spe­cif­ic op­er­a­tions. At its core, the func­tion es­tab­lish­es a pro­tect­ed com­mu­ni­ca­tion chan­nel be­tween user-mode ap­pli­ca­tions and tele­pho­ny hard­ware dri­vers through an ad­vanced han­dle man­age­ment sys­tem that now in­cor­po­rates Win­dows Im­ple­men­ta­tion Li­brary (WIL) fea­ture-gat­ed se­cu­ri­ty en­hance­ments. When the WIL fea­ture flag is en­abled (via wil::details::FeatureImpl<>::__private_IsEnabled), the func­tion ac­ti­vates its se­cure path which uti­lizes an aug­ment­ed CreateHandleTableEntry that ac­cepts both the pa­ra­me­ter point­er and size (dw­Size), en­abling ro­bust bounds val­i­da­tion and pre­vent­ing mem­o­ry cor­rup­tion at­tacks through pre­cise han­dle al­lo­ca­tion.

lineDe­vSpe­cif­ic Sum­ma­ry

The un­patched lineDevSpecific func­tion con­tained crit­i­cal se­cu­ri­ty vul­ner­a­bil­i­ties due to un­val­i­dat­ed han­dle cre­ation and miss­ing size checks, where the ba­sic CreateHandleTableEntry op­er­a­tion ac­cept­ed pa­ra­me­ters with­out ver­i­fy­ing buffer sizes, cre­at­ing po­ten­tial mem­o­ry cor­rup­tion risks. The patched ver­sion in­tro­duces a ro­bust se­cu­ri­ty up­grade through Win­dows Im­ple­men­ta­tion Li­brary (WIL) in­te­gra­tion, im­ple­ment­ing a dual-path ar­chi­tec­ture that ac­ti­vates size-aware han­dle cre­ation (CreateHandleTableEntry with dwSize pa­ra­me­ter) when se­cu­ri­ty fea­tures are en­abled, while main­tain­ing lega­cy com­pat­i­bil­i­ty. This key en­hance­ment ad­dress­es buffer over­flow vul­ner­a­bil­i­ties by val­i­dat­ing pa­ra­me­ter sizes dur­ing han­dle al­lo­ca­tion, while pre­serv­ing the orig­i­nal stack cook­ie pro­tec­tion and asyn­chro­nous ex­e­cu­tion mod­el. The patch sys­tem­at­i­cal­ly elim­i­nates mem­o­ry cor­rup­tion risks through run­time-con­fig­urable bounds check­ing, demon­strat­ing Mi­crosoft’s se­cure-by-de­sign ap­proach to hard­en­ing tele­pho­ny APIs with­out break­ing ex­ist­ing func­tion­al­i­ty – main­tain­ing back­ward com­pat­i­bil­i­ty while adding cru­cial size val­i­da­tion and han­dle man­age­ment pro­tec­tions against ex­ploita­tion at­tempts.

Con­clu­sion

In this part, we per­formed patch diff­ing on Mi­crosoft Tele­pho­ny Ser­vices, fo­cus­ing on crit­i­cal vul­ner­a­bil­i­ties ad­dressed in the 2025 se­cu­ri­ty up­dates. By com­par­ing the un­patched and patched ver­sions of key func­tions such as GrowBuf, lineGatherDigitsWPostProcess, and lineDevSpecific, we un­cov­ered sig­nif­i­cant se­cu­ri­ty im­prove­ments. The patch­es in­tro­duced ro­bust mit­i­ga­tions against heap-based buffer over­flows and in­te­ger over­flows, in­cor­po­rat­ing WIL fea­ture flags for run­time-con­fig­urable pro­tec­tions, 64-bit arith­metic checks to pre­vent size cal­cu­la­tion over­flows, and strict bounds val­i­da­tion for mem­o­ry op­er­a­tions. These changes high­light Mi­crosoft’s com­mit­ment to hard­en­ing lega­cy com­po­nents while main­tain­ing back­ward com­pat­i­bil­i­ty. Un­der­stand­ing these patch­es not only re­veals the vul­ner­a­bil­i­ties they fix but also pro­vides valu­able in­sights into se­cure cod­ing prac­tices for mem­o­ry man­age­ment in com­plex sys­tems. Stay tuned for Part 3, where we’ll dive deep­er into the oth­er func­tions and fol­low­ing by the ex­ploita­tion tech­niques and proof-of-con­cept de­vel­op­ment for these vul­ner­a­bil­i­ties!