CVE-2026-4177: Perl YAML::Syck Buffer Overflow Vulnerability

CVE-2026-4177 Overview

CVE-2026-4177 is a critical heap buffer overflow vulnerability affecting YAML::Syck versions through 1.36 for Perl. The vulnerability includes multiple security issues in the C-layer of the YAML parser and emitter, most notably a heap buffer overflow that occurs when class names exceed the initial 512-byte allocation in the YAML emitter.

Additionally, the library contains several other security weaknesses: an out-of-bounds read in the base64 decoder when processing trailing newlines, data corruption via strtok mutating n->type_id in place affecting shared node data, and a memory leak in syck_hdlr_add_anchor when a node already has an anchor assigned.

Critical Impact

Successful exploitation could allow remote attackers to execute arbitrary code or cause denial of service by triggering the heap buffer overflow through maliciously crafted YAML input with oversized class names.

Affected Products

• YAML::Syck versions through 1.36 for Perl

Discovery Timeline

• 2026-03-16 - CVE-2026-4177 published to NVD
• 2026-03-17 - Last updated in NVD database

Technical Details for CVE-2026-4177

Vulnerability Analysis

The primary vulnerability in CVE-2026-4177 is a heap buffer overflow (CWE-122) in the YAML emitter component of the YAML::Syck library. The emitter allocates a fixed 512-byte buffer for storing class names during YAML serialization. When processing YAML content with class names exceeding this limit, the emitter writes beyond the allocated buffer, corrupting adjacent heap memory.

The secondary issues compound the security risk. The base64 decoder fails to properly validate buffer boundaries when encountering trailing newline characters, leading to out-of-bounds read operations. The use of strtok on n->type_id causes in-place string mutation, which corrupts shared node data structures. Finally, the anchor handling code leaks memory when attempting to assign an anchor to a node that already has one, as the incoming anchor string is not freed on early return.

Root Cause

The root cause stems from insufficient boundary checking in C-layer memory operations. The emitter's fixed 512-byte buffer allocation does not account for arbitrarily long class names, and no validation occurs before writing data. Similarly, the base64 decoder's loop condition for skipping whitespace characters (\r and \n) did not verify that the pointer remained within bounds before accessing memory.

Attack Vector

The vulnerability is network-accessible, allowing remote attackers to craft malicious YAML input that triggers the heap overflow. An attacker could deliver the malicious YAML through any application that processes untrusted YAML data using the affected YAML::Syck library. This includes web applications accepting YAML uploads, configuration parsers processing remote data, or any service deserializing YAML from network sources.

The following patch excerpt shows the fix for the base64 decoder out-of-bounds read issue:

         }
     }
     while (s < send) {
-        while (s[0] == '\r' || s[0] == '\n') { s++; }
+        while (s < send && (s[0] == '\r' || s[0] == '\n')) { s++; }
+        if (s >= send) break;
         if ((a = b64_xtable[(int)s[0]]) == -1) break;
         if ((b = b64_xtable[(int)s[1]]) == -1) break;
         if ((c = b64_xtable[(int)s[2]]) == -1) break;

Source: GitHub Patch Commit

The memory leak fix in the anchor handler:

          * Happens if you have two anchors after each other or an anchor
          * before an alias
          * */
+        S_FREE(a);
         return n;
     }
     n->anchor = a;

Source: GitHub Patch Commit

Detection Methods for CVE-2026-4177

Indicators of Compromise

• Application crashes or segmentation faults when processing YAML files with unusually long class names or type identifiers
• Memory corruption indicators such as unexpected process termination or abnormal memory allocation patterns in Perl applications using YAML::Syck
• Error logs showing heap corruption warnings or buffer overrun detections from security tools

Detection Strategies

• Scan for YAML::Syck installations at version 1.36 or earlier using package management tools (cpan -D YAML::Syck or checking $YAML::Syck::VERSION)
• Implement runtime monitoring for heap corruption signals (SIGSEGV, SIGABRT) in processes using the vulnerable library
• Deploy application-layer firewalls or input validation to detect YAML payloads containing excessively long class names

Monitoring Recommendations

• Monitor application logs for segmentation faults or memory-related errors in services processing YAML data
• Implement file integrity monitoring on systems where YAML::Syck is installed to detect unauthorized library modifications
• Review network traffic for unusually large YAML payloads being submitted to applications

How to Mitigate CVE-2026-4177

Immediate Actions Required

• Upgrade YAML::Syck to version 1.37_01 or later which contains the security fixes
• Audit applications to identify all instances where YAML::Syck processes untrusted input
• Implement input size validation to reject YAML documents with class names exceeding reasonable lengths

Patch Information

The security patches addressing all four C-layer audit findings are available in the GitHub Patch Commit. The patched version 1.37_01 is documented in the MetaCPAN Change Log. Additional details were disclosed on the OpenWall OSS Security List.

Workarounds

• Restrict YAML::Syck usage to trusted input sources only until patching is complete
• Consider switching to alternative YAML parsing libraries such as YAML::XS or YAML::PP that are not affected by this vulnerability
• Implement application-level input validation to limit the size and complexity of YAML documents before parsing

# Upgrade YAML::Syck via CPAN
cpan YAML::Syck

# Verify installation version
perl -MYAML::Syck -e 'print $YAML::Syck::VERSION'