Out Of Bounds Write in Echo Go with Api Keys

An Out Of Bounds Write in an Echo Go service occurs when a handler writes data beyond the allocated memory of a buffer or slice. When API keys are handled naively—e.g., bound to request context, copied into byte buffers, or used to index arrays—the combination of key processing and unchecked memory operations can expose or trigger the vulnerability.

Consider an Echo handler that uses an API key to derive a lookup index, such as hashing the key and using the result to index a fixed-size array:

keys := []string{"ak_live_abc123", "ak_test_xyz789"}
func nextKey(c echo.Context) error {
    key := c.Request().Header.Get("X-API-Key")
    idx := int(hashKey(key)) % len(keys)
    // Risk: if hashKey produces an unexpected large value or key is malformed,
    // and the logic that uses idx does not validate bounds before writing,
    // an Out Of Bounds Write may occur in downstream slices or buffers.
    buf := make([]byte, 10)
    copy(buf, key) // potential overflow if key is longer than len(buf)
    return c.JSON(http.StatusOK, H{"index": idx})
}

In this example, the API key enters via HTTP header, influences an index calculation, and is copied into a fixed-size buffer. If the key length exceeds the buffer, copy writes past the end of buf. Even without direct user-controlled slice resizing, an attacker can supply a key crafted to produce an index that bypasses intended logical boundaries, leading to corrupted memory if the application uses CGo or interoperates with systems that do not enforce bounds checks.

Another scenario involves using the API key to control iteration or offsets in byte slices, such as when parsing tokens or constructing response buffers:

type session struct {
    data []byte
}
func handle(c echo.Context) error {
    key := c.Param("key")
    offset := calculateOffset(key)
    sess := session{data: make([]byte, 256)}
    // Risk: if calculateOffset(key) returns a value beyond 255,
    // the write below may occur outside sess.data.
    sess.data[offset] = 0x01
    return c.NoContent(http.StatusOK)
}

Here, the API key influences offset, which indexes into a fixed 256-byte slice. If calculateOffset is not constrained, an out-of-bounds write can corrupt adjacent memory. In Go, this may manifest as panics or, in environments with unsafe memory interactions, as exploitable behavior. The vulnerability is not in Echo itself but in how the application binds API key usage to memory operations without strict validation.

Middleware that inspects or transforms API keys can also introduce risk if buffers are reused or if key-derived values feed into dynamic structures without capacity checks:

func keyValidator(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        raw := c.Request().Header.Get("X-API-Key")
        normalized := strings.ToUpper(raw)
        temp := make([]byte, 32)
        copy(temp, normalized)
        // If normalized exceeds 32 bytes, copy writes out of bounds.
        c.Set("safeKey", temp)
        return next(c)
    }
}

To mitigate, ensure all writes respect slice length and that API key–derived indices are validated against container sizes before access.

Remediation focuses on validating API key length and derived indices before any memory operation, and avoiding direct use of key bytes in buffer copies.

1. Validate key length before copying into fixed-size buffers:

const maxKeyLen = 64
func safeCopyKey(c echo.Context) error {
    key := c.Request().Header.Get("X-API-Key")
    if len(key) > maxKeyLen {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid key length")
    }
    var buf [64]byte
    copy(buf[:], key)
    return c.JSON(http.StatusOK, H{"ok": true})
}

2. Constrain index calculations used for array or slice access:

func boundedIndex(c echo.Context) error {
    key := c.Request().Header.Get("X-API-Key")
    idx := int(hashKey(key)) % 100 // assume a 100-element pool
    if idx < 0 || idx >= 100 {
        return echo.NewHTTPError(http.StatusInternalServerError, "index error")
    }
    pool := make([]string, 100)
    // Safe: idx is within bounds
    pool[idx] = key
    return c.JSON(http.StatusOK, H{"index": idx})
}

3. Replace unsafe copies with length-aware slicing:

func processKey(c echo.Context) error {
    key := c.Request().Header.Get("X-API-Key")
    max := 256
    if len(key) > max {
        key = key[:max]
    }
    sess := session{data: make([]byte, 256)}
    offset := calculateOffset(key)
    if offset < 0 || offset >= len(sess.data) {
        return echo.NewHTTPError(http.StatusBadRequest, "offset out of range")
    }
    sess.data[offset] = 0x01
    return c.JSON(http.StatusOK, H{"offset": offset})
}

4. Use defensive copies when storing keys in context or middleware:

func keyValidator(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        raw := c.Request().Header.Get("X-API-Key")
        limit := 32
        if len(raw) > limit {
            raw = raw[:limit]
        }
        dst := make([]byte, limit)
        copy(dst, raw)
        c.Set("safeKey", dst)
        return next(c)
    }
}

These patterns ensure that API key usage does not introduce Out Of Bounds Write risks, keeping index derivation and buffer operations within safe memory boundaries.