CVE-2026-86143

In xmlIO in libxml2 before 2.15.4, an inconsistency in xmlOutputWriteCallback and xmlBufUse causes negative lengths to reach write callbacks, aka a lack of a check for integer overflow before calling writecallback. This has security relevance for many types of uses of that length value within a callback.

This library allows to manipulate XML files. It includes support to read, modify and write XML and HTML files. There is DTDs support this includes parsing and validation even with complex DtDs, either at parse time or later once the document has been modified. The output can be a simple SAX stream or and in-memory DOM like representations. In this case one can use the built-in XPath and XPointer implementation to select sub nodes or ranges. A flexible Input/Output mechanism is available, with existing HTTP and FTP modules and combined to an URI library.

Security Fix(es):

A flaw was found in libxml2. A local user or an attacker providing a specially crafted XML catalog can trigger a NULL pointer dereference during XML catalog parsing. This occurs when a nextCatalog element lacks its mandatory catalog attribute, leading to the application crashing and causing a Denial of Service (DoS).(CVE-2026-76781)

In libxml2 before 2.15.4, there is a heap-based buffer overflow in xmlXPtrEvalXPtrPart because of xmlXPtrEval xpointer length saturation.(CVE-2026-86142)

In xmlIO in libxml2 before 2.15.4, an inconsistency in xmlOutputWriteCallback and xmlBufUse causes negative lengths to reach write callbacks, aka a lack of a check for integer overflow before calling writecallback. This has security relevance for many types of uses of that length value within a callback.(CVE-2026-86143)

In xinclude in libxml2 before 2.15.4, xmlXIncludeProcess and xmlXIncludeProcessTree do not propagate parseFlags. This has security relevance for, for example, the XML_PARSE_NONET flag, if (without it) a custom resource loader accesses the internet and triggers XML external entity injection, SSRF, or a denial of service (e.g., for an attacker-controlled internet resource that is intentionally slow).(CVE-2026-86144)

This library allows to manipulate XML files. It includes support to read, modify and write XML and HTML files. There is DTDs support this includes parsing and validation even with complex DtDs, either at parse time or later once the document has been modified. The output can be a simple SAX stream or and in-memory DOM like representations. In this case one can use the built-in XPath and XPointer implementation to select sub nodes or ranges. A flexible Input/Output mechanism is available, with existing HTTP and FTP modules and combined to an URI library.

Security Fix(es):

A flaw was found in libxml2. A local user or an attacker providing a specially crafted XML catalog can trigger a NULL pointer dereference during XML catalog parsing. This occurs when a nextCatalog element lacks its mandatory catalog attribute, leading to the application crashing and causing a Denial of Service (DoS).(CVE-2026-76781)

In libxml2 before 2.15.4, there is a heap-based buffer overflow in xmlXPtrEvalXPtrPart because of xmlXPtrEval xpointer length saturation.(CVE-2026-86142)

In xmlIO in libxml2 before 2.15.4, an inconsistency in xmlOutputWriteCallback and xmlBufUse causes negative lengths to reach write callbacks, aka a lack of a check for integer overflow before calling writecallback. This has security relevance for many types of uses of that length value within a callback.(CVE-2026-86143)

In xinclude in libxml2 before 2.15.4, xmlXIncludeProcess and xmlXIncludeProcessTree do not propagate parseFlags. This has security relevance for, for example, the XML_PARSE_NONET flag, if (without it) a custom resource loader accesses the internet and triggers XML external entity injection, SSRF, or a denial of service (e.g., for an attacker-controlled internet resource that is intentionally slow).(CVE-2026-86144)

This library allows to manipulate XML files. It includes support to read, modify and write XML and HTML files. There is DTDs support this includes parsing and validation even with complex DtDs, either at parse time or later once the document has been modified. The output can be a simple SAX stream or and in-memory DOM like representations. In this case one can use the built-in XPath and XPointer implementation to select sub nodes or ranges. A flexible Input/Output mechanism is available, with existing HTTP and FTP modules and combined to an URI library.

Security Fix(es):

A flaw was found in libxml2. A local user or an attacker providing a specially crafted XML catalog can trigger a NULL pointer dereference during XML catalog parsing. This occurs when a nextCatalog element lacks its mandatory catalog attribute, leading to the application crashing and causing a Denial of Service (DoS).(CVE-2026-76781)

In libxml2 before 2.15.4, there is a heap-based buffer overflow in xmlXPtrEvalXPtrPart because of xmlXPtrEval xpointer length saturation.(CVE-2026-86142)

In xmlIO in libxml2 before 2.15.4, an inconsistency in xmlOutputWriteCallback and xmlBufUse causes negative lengths to reach write callbacks, aka a lack of a check for integer overflow before calling writecallback. This has security relevance for many types of uses of that length value within a callback.(CVE-2026-86143)

In xinclude in libxml2 before 2.15.4, xmlXIncludeProcess and xmlXIncludeProcessTree do not propagate parseFlags. This has security relevance for, for example, the XML_PARSE_NONET flag, if (without it) a custom resource loader accesses the internet and triggers XML external entity injection, SSRF, or a denial of service (e.g., for an attacker-controlled internet resource that is intentionally slow).(CVE-2026-86144)

This library allows to manipulate XML files. It includes support to read, modify and write XML and HTML files. There is DTDs support this includes parsing and validation even with complex DtDs, either at parse time or later once the document has been modified. The output can be a simple SAX stream or and in-memory DOM like representations. In this case one can use the built-in XPath and XPointer implementation to select sub nodes or ranges. A flexible Input/Output mechanism is available, with existing HTTP and FTP modules and combined to an URI library.

Security Fix(es):

A flaw was found in libxml2. A local user or an attacker providing a specially crafted XML catalog can trigger a NULL pointer dereference during XML catalog parsing. This occurs when a nextCatalog element lacks its mandatory catalog attribute, leading to the application crashing and causing a Denial of Service (DoS).(CVE-2026-76781)

In libxml2 before 2.15.4, there is a heap-based buffer overflow in xmlXPtrEvalXPtrPart because of xmlXPtrEval xpointer length saturation.(CVE-2026-86142)

In xmlIO in libxml2 before 2.15.4, an inconsistency in xmlOutputWriteCallback and xmlBufUse causes negative lengths to reach write callbacks, aka a lack of a check for integer overflow before calling writecallback. This has security relevance for many types of uses of that length value within a callback.(CVE-2026-86143)

In xinclude in libxml2 before 2.15.4, xmlXIncludeProcess and xmlXIncludeProcessTree do not propagate parseFlags. This has security relevance for, for example, the XML_PARSE_NONET flag, if (without it) a custom resource loader accesses the internet and triggers XML external entity injection, SSRF, or a denial of service (e.g., for an attacker-controlled internet resource that is intentionally slow).(CVE-2026-86144)

The libxml2 library is a development toolbox providing the implementation of various XML standards.

Security Fix(es):

• libxml2: libxml2: Arbitrary code execution via heap-based buffer overflow (CVE-2026-86138)

• libxml2: libxml2: Data integrity issues due to integer overflow in write callbacks (CVE-2026-86143)

• libxml2: libxml2: Arbitrary code execution via stack-based buffer overflow in xmlSnprintfElements (CVE-2026-86140)

• libxml2: libxml2: Heap-based buffer overflow in xmlXPtrEval due to xpointer length saturation (CVE-2026-86142)

• libxml2: libxml2: Information disclosure, SSRF, or denial of service due to improper parseFlags propagation. (CVE-2026-86144)

• libxml2: double-free/UAF in libxml2 Python bindings (CVE-2026-74860)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The libxml2 library is a development toolbox providing the implementation of various XML standards.

Security Fix(es):

• libxml2: libxml2: Arbitrary code execution via heap-based buffer overflow (CVE-2026-86138)

• libxml2: libxml2: Data integrity issues due to integer overflow in write callbacks (CVE-2026-86143)

• libxml2: libxml2: Arbitrary code execution via stack-based buffer overflow in xmlSnprintfElements (CVE-2026-86140)

• libxml2: libxml2: Heap-based buffer overflow in xmlXPtrEval due to xpointer length saturation (CVE-2026-86142)

• libxml2: libxml2: Information disclosure, SSRF, or denial of service due to improper parseFlags propagation. (CVE-2026-86144)

• libxml2: double-free/UAF in libxml2 Python bindings (CVE-2026-74860)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The libxml2 library is a development toolbox providing the implementation of various XML standards.

Security Fix(es):

• libxml2: libxml2: Arbitrary code execution via heap-based buffer overflow (CVE-2026-86138)

• libxml2: libxml2: Data integrity issues due to integer overflow in write callbacks (CVE-2026-86143)

• libxml2: libxml2: Arbitrary code execution via stack-based buffer overflow in xmlSnprintfElements (CVE-2026-86140)

• libxml2: libxml2: Information disclosure, SSRF, or denial of service due to improper parseFlags propagation. (CVE-2026-86144)

• libxml2: double-free/UAF in libxml2 Python bindings (CVE-2026-74860)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

In xmlIO in libxml2 before 2.15.4, an inconsistency in xmlOutputWriteCallback and xmlBufUse causes negative lengths to reach write callbacks, aka a lack of a check for integer overflow before calling writecallback. This has security relevance for many types of uses of that length value within a callback.

The libxml2 library is a development toolbox providing the implementation of various XML standards.

Security Fix(es):

• libxml2: libxml2: Arbitrary code execution via heap-based buffer overflow (CVE-2026-86138)
• libxml2: libxml2: Data integrity issues due to integer overflow in write callbacks (CVE-2026-86143)
• libxml2: libxml2: Arbitrary code execution via stack-based buffer overflow in xmlSnprintfElements (CVE-2026-86140)
• libxml2: libxml2: Heap-based buffer overflow in xmlXPtrEval due to xpointer length saturation (CVE-2026-86142)
• libxml2: libxml2: Information disclosure, SSRF, or denial of service due to improper parseFlags propagation. (CVE-2026-86144)
• libxml2: double-free/UAF in libxml2 Python bindings (CVE-2026-74860)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The libxml2 library is a development toolbox providing the implementation of various XML standards.

Security Fix(es):

• libxml2: libxml2: Arbitrary code execution via heap-based buffer overflow (CVE-2026-86138)
• libxml2: libxml2: Data integrity issues due to integer overflow in write callbacks (CVE-2026-86143)
• libxml2: libxml2: Arbitrary code execution via stack-based buffer overflow in xmlSnprintfElements (CVE-2026-86140)
• libxml2: libxml2: Heap-based buffer overflow in xmlXPtrEval due to xpointer length saturation (CVE-2026-86142)
• libxml2: libxml2: Information disclosure, SSRF, or denial of service due to improper parseFlags propagation. (CVE-2026-86144)
• libxml2: double-free/UAF in libxml2 Python bindings (CVE-2026-74860)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

The libxml2 library is a development toolbox providing the implementation of various XML standards.

Security Fix(es):

• libxml2: libxml2: Arbitrary code execution via heap-based buffer overflow (CVE-2026-86138)
• libxml2: libxml2: Data integrity issues due to integer overflow in write callbacks (CVE-2026-86143)
• libxml2: libxml2: Arbitrary code execution via stack-based buffer overflow in xmlSnprintfElements (CVE-2026-86140)
• libxml2: libxml2: Information disclosure, SSRF, or denial of service due to improper parseFlags propagation. (CVE-2026-86144)
• libxml2: double-free/UAF in libxml2 Python bindings (CVE-2026-74860)

For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section.

In xmlIO in libxml2 before 2.15.4, an inconsistency in xmlOutputWriteCallback and xmlBufUse causes negative lengths to reach write callbacks, aka a lack of a check for integer overflow before calling writecallback. This has security relevance for many types of uses of that length value within a callback.

• SECURITY UPDATE: use-after-free in the DTD parser reachable through parameter-entity references
  • debian/patches/CVE-2026-6653.patch: drop the post-inputPush XML_PARSER_EOF early return from xmlPushInput() in parser.c. It reported an already-halted parser as a push failure after ownership of the input had been transferred, so xmlParsePEReference() freed the input still installed as ctxt->input and xmlParseInternalSubset() read through the dangling pointer. The flaw is introduced by the CVE-2021-3541 backport, not by the shipped version.
  • CVE-2026-6653

• CVE-2026-86143: reject output buffer lengths that do not fit in an int before calling writecallback in xmlOutputBufferWrite, xmlOutputBufferWriteEscape and xmlOutputBufferFlush, so that a negative length can no longer reach a write callback
• CVE-2026-86144: propagate the document parse flags in xmlXIncludeProcess, xmlXIncludeProcessTree and xmlXIncludeLoadTxt so that options such as XML_PARSE_NONET are honoured during XInclude resolution

• CVE-2026-86143: guard the size_t buffer length against INT_MAX before it is narrowed to the int passed to writecallback in xmlOutputBufferWrite(), xmlOutputBufferWriteEscape() and xmlOutputBufferFlush() (xmlIO.c), so a negative length can no longer reach an output callback
• CVE-2026-86144: propagate the document's parseFlags in xmlXIncludeProcess() and xmlXIncludeProcessTree() and apply them to the parse="text" sub-context in xmlXIncludeLoadTxt() (xinclude.c), so XML_PARSE_NONET is honoured during XInclude resolution

• CVE-2026-86143: guard the size_t buffer length against INT_MAX before it is narrowed to the int passed to writecallback in xmlOutputBufferWrite(), xmlOutputBufferWriteEscape() and xmlOutputBufferFlush() (xmlIO.c), so a negative length can no longer reach an output callback
• CVE-2026-86144: propagate the document's parseFlags in xmlXIncludeProcess() and xmlXIncludeProcessTree() and apply them to the parse="text" sub-context in xmlXIncludeLoadTxt() (xinclude.c), so XML_PARSE_NONET is honoured during XInclude resolution

• CVE-2026-86143: guard the size_t buffer length against INT_MAX before it is narrowed to the int passed to writecallback in xmlOutputBufferWrite(), xmlOutputBufferWriteEscape() and xmlOutputBufferFlush() (xmlIO.c), so a negative length can no longer reach an output callback
• CVE-2026-86144: propagate the document's parseFlags in xmlXIncludeProcess() and xmlXIncludeProcessTree() and apply them to the parse="text" sub-context in xmlXIncludeLoadTxt() (xinclude.c), so XML_PARSE_NONET is honoured during XInclude resolution

• SECURITY UPDATE: integer overflow leading to heap buffer overflow in xmlDictAddQString()
  • debian/patches/CVE-2026-86138.patch: add SIZE_MAX-based overflow guards to the pool size computation in xmlDictAddQString() in dict.c, plus a SIZE_MAX fallback define, so an oversized prefix and name can no longer wrap the allocation size and let the following memcpy() write past the dictionary string pool
  • CVE-2026-86138

• SECURITY UPDATE: stack buffer overflow in xmlSnprintfElements()
  • debian/patches/CVE-2026-86140.patch: add bounds checks around the parenthesis writes in xmlSnprintfElements() in valid.c
  • CVE-2026-86140
• SECURITY UPDATE: NULL pointer dereference in xmlRegNewParserCtxt() after a failed allocation
  • debian/patches/CVE-2026-86141.patch: check the xmlStrdup() result in xmlRegNewParserCtxt() in xmlregexp.c so a failed allocation cannot leave a NULL parser cursor
  • CVE-2026-86141
• SECURITY UPDATE: heap buffer overflow in xmlXPtrEvalXPtrPart() on oversized xpointer() expressions
  • debian/patches/CVE-2026-86142.patch: make xmlStrlen() in xmlstring.c saturate above INT_MAX and reject the saturated length in xmlXPtrEvalXPtrPart() in xpointer.c before sizing the buffer
  • CVE-2026-86142
• SECURITY UPDATE: integer overflow before the output write callback
  • debian/patches/CVE-2026-86143.patch: check for integer overflow before passing a buffer length to the write callback in xmlIO.c
  • CVE-2026-86143
• SECURITY UPDATE: parse flags not propagated by xmlXIncludeProcess() and xmlXIncludeProcessTree()
  • debian/patches/CVE-2026-86144.patch: propagate the document parse flags in xmlXIncludeProcess() and xmlXIncludeProcessTree() in xinclude.c
  • CVE-2026-86144

• SECURITY UPDATE: heap buffer overflow in xmlXPtrEvalXPtrPart() on oversized xpointer() expressions
  • debian/patches/CVE-2026-86142.patch: make xmlStrlen() in xmlstring.c saturate above INT_MAX and reject the saturated length in xmlXPtrEvalXPtrPart() in xpointer.c before sizing the buffer
  • CVE-2026-86142
• SECURITY UPDATE: integer overflow before the output write callback
  • debian/patches/CVE-2026-86143.patch: check for integer overflow before passing a buffer length to the write callback in xmlIO.c
  • CVE-2026-86143
• SECURITY UPDATE: parse flags not propagated by xmlXIncludeProcess() and xmlXIncludeProcessTree()
  • debian/patches/CVE-2026-86144.patch: propagate the document parse flags in xmlXIncludeProcess() and xmlXIncludeProcessTree() in xinclude.c
  • CVE-2026-86144

• CVE-2026-86138: add overflow checks to xmlDictAddQString to prevent an integer overflow and heap buffer overflow
• CVE-2026-86140: bound the writes in xmlSnprintfElements to prevent a stack buffer overflow
• CVE-2026-86142: make xmlStrlen saturate above INT_MAX and reject the saturated length in xmlXPtrEvalXPtrPart to prevent a heap buffer overflow
• CVE-2026-86143: reject buffer lengths at or above INT_MAX before calling the output write callback to prevent an integer overflow
• CVE-2026-86144: propagate parseFlags in xmlXIncludeProcess and xmlXIncludeProcessTree so parser flags such as XML_PARSE_NONET are honoured

• CVE-2026-86140: add the missing bounds checks around the two unguarded strcat() calls in xmlSnprintfElements(), the sibling of the function fixed for CVE-2025-24928
• CVE-2026-86142: reject a saturated xmlStrlen() result in xmlXPtrEvalXPtrPart() before it is used to size the XPointer buffer
• CVE-2026-86143: check the xmlBufUse() length against INT_MAX before narrowing it to the int parameter of the output writecallback, at all six call sites
• CVE-2026-86144: propagate the document parseFlags through xmlXIncludeProcess(), xmlXIncludeProcessTree() and xmlXIncludeLoadTxt() so XML_PARSE_NONET is honoured during XInclude resolution

• CVE-2026-86140: add bounds checks around the parenthesis writes in xmlSnprintfElements
• CVE-2026-86141: check the xmlStrdup result in xmlRegNewParserCtxt so a failed allocation cannot leave a NULL parser cursor
• CVE-2026-86142: make xmlStrlen saturate above INT_MAX and reject the saturated length in xmlXPtrEvalXPtrPart before sizing the buffer
• CVE-2026-86143: check for integer overflow before passing a buffer length to the output write callback
• CVE-2026-86144: propagate the document parse flags in xmlXIncludeProcess and xmlXIncludeProcessTree

• CVE-2026-86140: add bounds checks around the parenthesis writes in xmlSnprintfElements
• CVE-2026-86141: check the xmlStrdup result in xmlRegNewParserCtxt so a failed allocation cannot leave a NULL parser cursor
• CVE-2026-86142: make xmlStrlen saturate above INT_MAX and reject the saturated length in xmlXPtrEvalXPtrPart before sizing the buffer
• CVE-2026-86143: check for integer overflow before passing a buffer length to the output write callback
• CVE-2026-86144: propagate the document parse flags in xmlXIncludeProcess and xmlXIncludeProcessTree

• CVE-2026-86140: add bounds checks around the parenthesis writes in xmlSnprintfElements
• CVE-2026-86141: check the xmlStrdup result in xmlRegNewParserCtxt so a failed allocation cannot leave a NULL parser cursor
• CVE-2026-86142: make xmlStrlen saturate above INT_MAX and reject the saturated length in xmlXPtrEvalXPtrPart before sizing the buffer
• CVE-2026-86143: check for integer overflow before passing a buffer length to the output write callback
• CVE-2026-86144: propagate the document parse flags in xmlXIncludeProcess and xmlXIncludeProcessTree

• CVE-2026-86143: reject buffer lengths at or above INT_MAX before calling the output write callback to prevent an integer overflow
• CVE-2026-86144: propagate parseFlags in xmlXIncludeProcess and xmlXIncludeProcessTree so parser flags such as XML_PARSE_NONET are honoured

The updated packages fix several security issues: Stack-Based Buffer Overflow in libxml2. (CVE-2026-11979) In libxml2 before 2.15.4, xmlFAParsePosCharGroup has an out-of-bounds read, aka an out-of-bounds read in the NXT macro in xmlregexp. (CVE-2026-86137) In libxml2 before 2.15.4, xmlDictAddQString in dict.c has an integer overflow and resultant heap-based buffer overflow. (CVE-2026-86138) In libxml2 before 2.15.4, xmlURIEscapeStr in uri.c has an integer overflow. (CVE-2026-86139) In libxml2 before 2.15.4, xmlSnprintfElements in valid.c has a strcat stack-based buffer overflow. (CVE-2026-86140) xmlregexp in libxml2 before 2.15.4 has a NULL pointer dereference in xmlRegNewParserCtxt after a strdup failure, i.e., it does not calculate a string length after NULL checking. (CVE-2026-86141) In libxml2 before 2.15.4, there is a heap-based buffer overflow in xmlXPtrEvalXPtrPart because of xmlXPtrEval xpointer length saturation. (CVE-2026-86142) In xmlIO in libxml2 before 2.15.4, an inconsistency in xmlOutputWriteCallback and xmlBufUse causes negative lengths to reach write callbacks, aka a lack of a check for integer overflow before calling writecallback. This has security relevance for many types of uses of that length value within a callback. (CVE-2026-86143) In xinclude in libxml2 before 2.15.4, xmlXIncludeProcess and xmlXIncludeProcessTree do not propagate parseFlags. This has security relevance for, for example, the XML_PARSE_NONET flag, if (without it) a custom resource loader accesses the internet and triggers XML external entity injection, SSRF, or a denial of service (e.g., for an attacker-controlled internet resource that is intentionally slow). (CVE-2026-86144)

Root has patched CVE-2026-86143 in the libxml2 package for Root:Debian:12. Multiple fixed versions available.

Yirou Yang discovered that libxml2 incorrectly handled certain XML catalogs. If a user or automated system was tricked into processing a specially crafted XML catalog, an attacker could possibly use this issue to cause libxml2 to crash, resulting in a denial of service. (CVE-2026-76781)

It was discovered that libxml2 incorrectly handled certain large qualified names, leading to a heap-based buffer overflow. An attacker could possibly use this issue to cause libxml2 to crash, resulting in a denial of service, or possibly execute arbitrary code. (CVE-2026-86138)

It was discovered that libxml2 incorrectly handled escaping certain large URI strings. An attacker could possibly use this issue to cause libxml2 to use excessive resources, leading to a denial of service. This issue only affected Ubuntu 26.04 LTS. (CVE-2026-86139)

Xudong Cao and Meng Xu discovered that libxml2 incorrectly handled certain large XPointer expressions, leading to a heap-based buffer overflow. An attacker could possibly use this issue to cause libxml2 to crash, resulting in a denial of service, or possibly execute arbitrary code. (CVE-2026-86142)

Xudong Cao and Meng Xu discovered that libxml2 did not check for integer overflows before passing output lengths to write callbacks. An attacker could possibly use this issue to cause an application using libxml2 to crash, resulting in a denial of service. (CVE-2026-86143)

It was discovered that libxml2 did not apply parser options, such as disabling network access, when processing XInclude directives under certain circumstances. An attacker could possibly use this issue to perform XML external entity injection or server-side request forgery attacks, or cause a denial of service. (CVE-2026-86144)